Multi-Frequency (MF) tones are used extensively in telecommunications. Examples of MF tones are DTMF and R2. Typical prior art tone detectors include a pre-filter for suppressing out-of-band frequencies, an analysis filter for determining the energy of the input signal at each multi-frequency tone of interest, and a decision logic block for interpreting the multi-frequency energy values output from the analysis filter and in response performing a pass/fail test for determining the presence or absence of a tone at each multi-frequency tone of interest.
Prior art analysis filters have been implemented using standard digital filtering techniques. The analysis window for such prior art digital filters must be carefully chosen. Rectangular windows are often used for simplicity. However, the leakage of energies in the side lobes of the rectangular window is such that other MF frequencies may be incorrectly detected. Also, fast roll-off characteristics (i.e. high order) are usually imposed in order to fail any tone that deviates excessively from its nominal value.